Introduction: This study aimed to investigate capabities of MCNPX monte carlo code for calculations of average absorbed dose in a breast phantom during mammography examination. Also, the effect of tube voltage and breast thickness on absorbed dose was determined by using Monte carlo method. Materials & Methods: In this study, the average absorbed dose values were investigated by using MCNPX (version 2.4.0) Monte Carlo code. Validation of modeled simulation setup has been obtained by calculating the HVL values of Al attenuator material for different tube voltage (26 to 32 kV ). Obtained HVL values were found to be comparable with experimental results and available Monte Carlo results. Thus, validated simulation setup has been used for investigation of average absorbed dose in a %50-%50 grandular-adipose breast phantom. Results: Our calculated results was consistent with the previous studies and our MC model was validated. The results showed that changing tube voltage from 26 to 32 kV leads to about 4 times increase in breast dose. While increasing breast thick ness from 2 to 5 cm results in 1.6 times higer dose to breast. Conclusion: It can concluded that the breast recieved dose depends strongly beam quality and breast thickness. Our modeling using MCNPX can be used future dosimteric studies concering breast dose investigations in mamograpgy examinations.